The invention relates to the application of refractory borides to carbon-based components of cells for the production of aluminium by electrolysis of alumna dissolved in a cryolite-based molten electrolyte, in particular carbon cathodes. The invention also relates to such cells having carbon-based components protected from the corrosive attacks of liquids and/or gaseous components of the electrolyte in the form of elements, ions or compounds by having refractory borides applied to their surfaces, as well as the use of these cells for the production of aluminium.
Aluminium is produced conventionally by the Hall-Hxc3xa9roult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperature up to around 950xc2x0 C. A Hall-Hxc3xa9roult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Hxc3xa9roult cells, a molten aluminium pool acts as the cathode. The carbon lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminium metal produced.
A major drawback of carbon as cathode material is that it is not wetted by aluminium. This necessitates maintaining a deep pool of aluminium (at least 100-250mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminium and, to avoid short-circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminium pool and the anode. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals or Refractory Hard Materials (RHM) such as TiB2 as cathode materials. TiB2 and other RHM""s are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool. Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the savings of energy by reducing the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,214,615, 3, 314, 876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
The non-acceptance of tiles and other methods of applying layers of TiB2 and other RHM materials on the surface of aluminium production cells is due to their lack of stability in the operating conditions, in addition to their cost. The failure of these materials is associated with penetration of the electrolyte when not perfectly wetted by aluminium, and attach by aluminium because of impurities in the RHM structure. In RHM pieces such as tiles, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminium metal and/or by cryolite. To combat disintegration, it has been proposed to use highly pure TiB2 powder to make materials containing less than 50 ppm oxygen. Such fabrication further increases the cost of the already-expensive materials. No cell utilizing TiB2 tiles as cathode is known to have operated for long periods without loss of adhesion of the tiles, or their disintegration. Other reasons for failure of RHM tiles have been the lack of mechanical strength and resistance to thermal shock.
Various types of TiB2 or RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the titanium diboride material and the carbon cathode block.
U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride and other Refractory Hard Metals to a conductive substrate such as graphite. But large differences in thermal expansion coefficients between the RHM tiles and the substrate cause problems.
Copending application Ser. No. 08/028,359 (MOL0516), the content whereof is incorporated herein by way of reference, provides a method of bonding bodies of Refractory Hard Material (RHM) or other refractory composites to carbon cathodes of aluminium protection cells using a colloidal slurry comprising particulate preformed RHM in a colloidal carrier selected from colloidal alumina, colloidal yttria and colloidal ceria as a glue between the bodies and the cathode or other component. The slurry is dried to bond the bodies to the cathode or other component, the dried slurry acting as a conductive thermally-matched glue which provides excellent bonding of the bodies to the cathode or other component.
PCT application PCT/EP93/00811 (MOL0508) discloses a method or producing a protective refractory coating on a substrate of, inter-alia, carbonaceous materials by applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants in a colloidal carrier, and initiating a micropyretic reaction. The micropyretic slurry optionally also contains some preformed refractory material, and the micropyretic slurry may be applied on a non-reactive sub-layer.
PCT application PCT/EP93/00810 (MOL0513) discloses a body of carbonaceous or other material for use in corrosive environments such as oxidising media or gaseous or liquid corrosive agents at elevated temperatures, coated with a protective surface coating which improves the resistance of the body to oxidation or corrosion and which may also enhance the bodies electrical conductivity and/or its electrochemical activity. This protective coatingxe2x80x94in particular silica-based coatingsxe2x80x94is applied from a colloidal slurry containing particulate reactant or non-reactant substances, or a mixture of particulate reactant and non-reactant substances, which when the body is heated to a sufficient elevated temperature form the protective coating by reaction sintering and/or sintering without reaction.
The invention aims to overcome the deficiencies of past attempts to utilize refractory materials in particular refractory borides as surface coatings on carbonaceous substrates, for protecting the substrates from the corrosive attacks of liquids and gases when used as cell components for aluminium production cells, especially for use as cathodes.
An object of the invention is to provide refractory boride coatings that are well adherent to the carbon-containing substrates, provide the required protection to the cell components and have the desired mechanical, physical, chemical, and electrochemical characteristics.
A further object is to provide coatings which are adherent to the carbon-containing substrates and protect the substrates efficiently from the corrosive attacks of liquids, fumes and gases existing or produced in aluminum production cells wherein all cell components have to be mechanically strong at the operating temperature and each one may have any additional required characteristic.
A specific object of the invention is to provide an easy-to-implement method of applying refractory borides to carbon-containing cell components to form a coating which can be consolidated by heat treatment before or during use of the cell component to improve its protection, which method employs refractory borides in a readily-available form.
In particular, an aluminium-wettable, refractory, electrically conductive, adherent boride coating has been developed to be applied to the surface of the cell cathode bottom made of carbonaceous material to protect such carbonaceous material from the attack of sodium and air which produces deformation of the cathode blocks and formation of dangerous nitrogen compounds such as cyanides.
By protecting the carbonaceous cell components from attack by NaF or other aggressive ingredients of the electrolyte, the cell efficiency is improved. Because NaF in the electrolyte no longer reacts with the carbon cell bottom and walls, the cell functions with a defined bath ratio without a need to replenish the electrolyte with NaF.
The aluminum-wettable refractory boride coating will also permit the elimination of the thick aluminium pool required to partially protect the carbon cathode, enabling the cell to operate with a drained cathode. Other coatings have been developed to protect the upper part of the carbonaceous cell wall and cell cover and anode current feeders and holders from the attack of fluoride fumes and oxidation by oxygen or air and the lower part from the attack by the cryolite-containing electrolyte (see in particular PCT application PCT/EP90/00810).
The protective effect of the coatings according to the invention is such as to enable the use of relatively inexpensive carbon-containing materials for the substrates. For instance, cheaper grades of graphite can be used instead of the more expensive anthracite forms of carbon, while providing improved resistance against the corrosive conditions in the cell environment.
The refractory boride coatings have the following attributes: excellent wettability by molten aluminium, excellent adherence to the carbon-containing substrates, inertness to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks without delamination from the anthracite-based carbon or other carbonaceous substrates, durability in the environment of an aluminium production cell, and ease of application and processing. The preferred coatings furthermore have a controlled microporosity and degree of penetration in the porous carbonaceous substrate, by having an adequate distribution of the particle sizes of the preformed refractory boride.
When these refractory boride coatings are applied to a carbon-based substrate, for instance of graphite or anthracite-based carbon used in an aluminium production cell in contact with the molten electrolyte and/or with molten aluminium, the coating protects the substrate against the ingress of cryolite and sodium and is in turn protected by the protective film of aluminium on the coating itself.
The refractory boride coatings find many applications on account of their excellent resistance, protection, and stability when exposed to the corrosive action of liquids and fumes existing in the cell or formed during electrolysis even when the temperature of operation is low as in the Low Temperature electrolysis process for the production of aluminium (see for example U.S. Pat. No. 4,681,671 and PCT application PCT/EP92/02666).
One main aspect of the invention is a slurry for the application of refractory hard metal boride to carbon-containing components of cells for the production of aluminium by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, to protect such components from attack by liquid and/or gaseous components of the electrolyte in the form of elements, ions or compounds, wherein the slurry is composed of pre-formed particulate refractory boride in a colloidal carrier.
It is essential to use colloids and mixtures of colloids for application of the coatings. Three types of colloidal processing are possible. The first involves the gelation of certain polysaccharide solutions. This, however, is relatively unimportant to this invention. The other two which involve colloids and metal organic compounds are relevant to this invention. These two involve the mixing of materials in a very fine scale. Colloids are defined as comprising a dispersed phase with at least one dimension between 0.5 nm (nanometer) and about 10 micrometers in a dispersion medium which in our case is a liquid. The magnitude of this dimension distinguishes colloids from bulk systems in the following way: (a) an extremely large surface area and (b) a significant percentage of molecules reside in the surface of colloidal systems. Up to 40% of molecules may reside on the surface. The colloidal systems which are important to this invention are both the thermodynamically stable lyophylic type (which include macromolecular systems such as polymers) and the kinetically stable lyophobic type (those that contain particles).
Insoluble oxides in aqueous suspension develop surface electric charges by surface hydroxylation followed by dissociation of surface hydroxyl groups. Typical equations could be:
M(OH)surface+H2O less than = greater than MOxe2x88x92 surface+H3O+
M(OH)surface+H2O less than = greater than M(OH2)+ surface+OHxe2x88x92
where M represents a metal or a complex cation.
Such surface charges and the London and Ven der Waals forces keep the particles from agglomerating. An adsorbed layer of material, polymer or surface active agent, modifies the interaction of particles in several ways. In the mixing process described below, we introduce particulate pre-formed refractory borides.
Colloids may form through cation hydrolysis. Many metal ions are subject to hydrolysis because of high electronic charge or charge density. Initial products of hydrolysis can condense and polymerize to form polyvalent metal or polynuclear ions, which are themselves colloidal. Charge and pH determine the ligands for central cations and the anion/cation ratio controls the degree of polymerization and stability of the suspension. The pH could vary from 014 14. A wide range of polynuclear cationic hydrolysis products may exist with charge from 2+ to 6+. Polynuclear anionic hydrolysis products could also have a wide range of charges.
The formation of colloids involves a starting material for example a reagent grade metal salt which is converted in a chemical process to a dispersible oxide which forms the colloidal solution on addition of dilute acid or water. Removal of water (drying) and/or removal of the anions from the colloidal solution produces a gel like product.
The colloidal carrierxe2x80x94usually colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate, and usually in an aqueous mediumxe2x80x94has been found to considerably improve the properties of the coating produced by non-reactive sintering.
The colloidal slurry contains particulate pre-formed refractory hard metal boride(s). Above 900xc2x0 C., sintering or consolidation may occur during exposure to the service conditions at the high temperature.
The constituents of the slurries are:
(a) A carrier, chosen from colloidal liquids which could be colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate, cerium acetate or mixtures thereof.
(b) A powder additive containing pre-formed refractory borides.
The colloid may be derived from colloid precursors and reagents which are solutions of at least one salt such as chlorides, sulfates, nitrates, chlorates, perchlorates or metal organic compounds such as alkoxides, formates, acetates of aluminium, silicon, yttrium, cerium, thorium zirconium, magnesium and lithium. These colloid precursors or colloid reagents can contain a chelating agent such as acetyl acetone or ethylacetoacetate. The aforesaid solutions of metal organic compounds, principally metal alkoxides, can be of the general formula M (OR) where M is a metal or complex cation, R is an alkyl chain and z is a number, preferably from 1 to 12.
The pre-formed particulate refractory boride is selected from borides of titanium, chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenum and cerium. The preferred particulate refractory boride is titanium diboride.
When choosing powder additives the particle size selection is of importance. It is preferable to choose particle size below 100 micrometers and to choose particle sizes which are varied such that the packing of particles is optimized. For example it is preferable to choose particle sizes extending over a range where the smallest particles are at least two times and preferably at least three times smaller than the large ones. Generally, the ratio of the particle sizes will be in the range from 2:1 to 15:1, usually from about 3:1 to 10:1, for instance a ratio of about 3:1 with large particles in the range 15 to 30 micrometers and small particles in the range 5 to 10 micrometers, or a ratio of about 10:1 with large particles in the range from 30 to 50 micrometers and small particles in the range from 3 to 5 micrometers. Usually, the preformed particulate metal boride has particles with sizes in the range from about 3 micrometers to about 50 micrometers.
The slurry usually contains 5-100 g of the preformed particulate refractory boride per 10 ml of colloid and the colloid has a dry colloid content corresponding to up to 50 weight % of the colloid plus liquid carrier, preferably from 10 to 20 weight %.
The colloid is contained in a liquid such as water which may further contain at least one compound selected from compounds of lithium, aluminum, cerium, sodium and potassium, for instance at least one compound of lithium and at least one compound of aluminum, see copending application Ser. No 08/034,283 (MOL0518), the contents whereof are incorporated herein by way of reference.
Another aspect of the invention is a method of protecting carbon-containing cathodes from the attack of cryolite, molten aluminum and sodium by applying a coating of colloids containing TiB2 or other refractory hard metal borides.
The invention provides a method of applying a refractory hard metal boride to a carbon-containing component of a cell for the production of aluminum, in particular by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, this method comprising applying to the surface of the component a slurry of particulate preformed refractory boride in a colloidal carrier as specified above, followed by drying, and by heat treatment before or after the component is installed in an aluminum production cell.
The method of application of the slurry involves painting (by brush or roller), dipping, spraying, or pouring the slurry onto the substrate and allowing for drying before another layer is added. The coating need not entirely dry before the application of the next layer. It is preferred to heat the coating with a suitable heat source so as to completely dry it and improve densification of the coating. Heating takes place preferably in air but could be in other oxidizing atmospheres or in inert or reducing atmospheres. A heat treatment in air at about 80-200xc2x0 C., for half an hour to several hours is usually sufficient.
The substrate may be treated by sand blasting or pickled with acids or fluxes such as cryolite or other combinations of fluorides and chlorides prior to the application of the coating. Similarly the substrate may be cleaned with an organic solvent such as acetone to remove oily products and other debris prior to the application of the coating. These treatments will enhance the bonding of the coatings to the carbon-containing substrate.
After coating the substrate by dipping, painting or spraying the slurry or combinations of such techniques in single or multi-layer coatings and drying, a final coat of the colloid alone may be applied lightly prior to use.
More generally, before or after application of the coating and before use, the body can be painted, sprayed, dipped or infiltrated with reagents and precursors, gels and/or colloids. For instance, before applying the slurry of particulate refractory boride in the colloidal carrier the carbonaceous component can be impregnated with e.g. a compound of lithium to improve the resistance to penetration by sodium, as described in copending application Ser. No. 08/028,384 (MOL0515) the contents whereof are incorporated herein by way of reference.
To assist rapid wetting of the components by molten aluminum, the refractory material coated on the substrate may be exposed to molten aluminum in the presence of a flux assisting penetration of aluminum into the refractory material, the flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof. Such treatment favors aluminization of the refractory coating by the penetration therein of aluminum.
The substrate of the component may be coated outside the aluminum production cell and the coated component than inserted into the cell. Alternatively, the component is part of a cell which is coated in the cell prior to operation. For instance, the component is part of a cell bottom formed by an exposed area of carbonaceous material. In this case, the slurry is preferably applied to the cell bottom in several layers with drying of each successive layer and final drying by means of a mobile heat source.
More generally, the invention also concerns a method of improving the resistance to oxidation or corrosion of a body of carbonaceous material for use in corrosive environments such as oxidizing media or gaseous or liquid agents at elevated temperatures, the body being in particular a component of an electrochemical cell for the production of aluminum, in particular by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, which component in use is exposed to a corrosive atmosphere, or to cryolite and/or to a product of electrolysis in the cell. This method comprises applying to the body a protective coating from a slurry of the preformed refractory boride in a colloidal carrier, followed by heating the body prior to or during use to a sufficient temperature to cause the boride to consolidate to form an adherent protective coating.
The invention also concerns cell components of aluminum production cells, in particular those which in use of the cell are exposed to contact with molten cryolite and/or molten aluminum. The cell component is for instance a cathode or forms part of a cathodic cell bottom.
Other cell components are those which in use are exposed to corrosive or oxidizing gas released in operation of the cell or present in the cell operating conditions, which components are protected from corrosion or oxidation by the refractory boride coating as set out above.
According to the invention, there is provided a carbon-containing component of a cell for the production of aluminum by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, which cell component is protected from attack by liquid and/or gaseous components of the electrolyte in the form of elements, ions or compound, by a coating of preformed particulate refractory hard metal boride in a dried colloid applied on the cell component from a slurry of the preformed particulate refractory hard metal boride in a colloidal carrier, as set out above.
The component may be current-carrying component for example a cathode, a cathode current feeder, an anode or an anode current feeder. Or the component may be a bipolar electrode coated on its cathode face, or on its anode face, or both.
The slurry-applied refractory boride coatings may have a thickness from about 150 micrometers to about 1500 micrometers, usually from about 200 to about 500 micrometers, depending on the number of applied layers, the particle size of the preformed boride, and the porosity of the carbon. Advantageously, by using graded boride particles including fine particles, the smaller boride particles penetrate into the pores of the carbon component and firmly anchor the coating. Typically, the boride may impregnate the carbon to a depth of about 50-200 micrometers. The colloid impregnates the carbon component so the dried colloid is dispersed through the carbon component.
The invention concerns in general the protection of components of electrochemical cells for the production of aluminum by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, which components in use are exposed to a corrosive atmosphere, or to a molten cryolite, and/or to a product of electrolysis in the cell. Such components are coated with a protective surface coating which improves the resistance of the components to oxidation or corrosion and which may also enhance the electrical conductivity and/or electrochemical activity. The protective coating is applied from a colloidal slurry containing particulate preformed refractory boride and dried. When the component is heated to a sufficient elevated temperature, prior to or upon insertion in the cell, a protective coating in formed by sintering or consolidation without reaction.
The invention also concerns a component of an aluminum production cell which is use is subjected to exposure to molten cryolite and/or to molten aluminum or corrosive fumes or gases, the component comprising a substrate of a carbonaceous material, coated with a refractory boride, of at least one of titanium, chromium, vanadium, zirconium, hafnium, niobium, tantalum, molybdenum and cerium or mixtures thereof, finely mixed with a refractory compound of at least one alumina, silica, yttria, ceria, thoria, zirconia, magnesia and lithia from a dried colloid.
The component is usually made of carbonaceous material selected from petroleum coke, metallurgical coke, anthracite, graphite, amorphous carbon, fulerene, low density carbon or mixtures thereof. Composite materials based on one or more of these forms of carbon with other materials may also be employed.
It is advantageous for the component to have a substrate of low-density carbon protected by the refractory boride, for example if the component is exposed to oxidizing gas released in operation of the cell, or also when the substrate is part of a cell bottom. Low density carbon embraces various types of relatively inexpensive forms of carbon which are relatively porous and very conductive, but hitherto could not be used successfully in the environment of aluminum production cells on account of the fact that they were subject to excessive corrosion or oxidation. Now it is possible by coating these low density carbons according to the invention, to make use of them in these cells instead of the more expensive high density anthracite and graphite, taking advantage of their excellent conductivity and low cost.
The substrate may consist of carbonaceous blocks that can be fitted together to form a cell bottom of an aluminum production cell, or packed carbonaceous particulate material forming a cell bottom, which acts to carry current to the cathodic pool if there is one, or to a thin layer of aluminum through the refractory boride coating in drained cells.
The component advantageously forms part of a cathode which the electrolysis current flows, the refractory boride coating forming a cathodic surface in contact with the cathodically-produced aluminum. For example, it is part of a drained cathode, the refractory boride coating forming the cathodic surface on which the aluminum is deposited cathodically, and the component being arranged usually upright or at a slope for the aluminum to drain from the cathodic surface.
The invention also relates to an aluminum production cell comprising a coated component as discussed above as well as a method of producing aluminum using such cells and methods of assembling and/or operating the cells.
Such cells may comprise a component which in operation of the cell is exposed to molten cryolite or aluminum, said component comprising a substrate of carbonaceous material and a coating of refractory boride, applied from a colloidal slurry as discussed above, wherein the product aluminum is in contact with the refractory boride coating on the component, which may be a cathode or forms part of a cathodic cell bottom.
The invention also concerns an aluminum production cell having a component which in operation of the cell is exposed to corrosive or oxidizing gas released in operation of the cell or present in the cell operating conditions, said component comprising a substrate of carbonaceous material, and a coating of refractory boride deposited from a colloidal slurry, as discussed above.
A method of operating the cells comprises:
producing a cell component which comprises a substrate of carbonaceous material and a protective coating of refractory boride, by applying to the substrate a slurry containing particulate preformed refractory boride in a colloidal carrier drying and optionally subjecting the component to heat treatment;
placing the coated component in the cell so the coating of refractory material will be contacted by the cathodically produced aluminum, and/or the molten electrolyte, and/or the anodically-released gases; and
operating the cell with the coating protecting the substrate from attack by the cathodically-produced aluminum, by the molten electrolyte and by the anodically-released gases with which it is in contact.
Operation of the cell is advantageously in a low temperature process, with the molten halide electrolyte containing dissolved alumina at a temperature below 900xc2x0 C., usually at a temperature from 680xc2x0 C. to 880xc2x0 C. The low temperature electrolyte may be a fluoride melt, a mixed fluoride-chloride melt or a chloride melt.
This low temperature process is operated at low current densities on account of the low alumina solubility. This necessitates the use of large anodes and corresponding large cathodes, exposing large areas of these materials to the corrosive conditions in the cell, such large exposed areas being well protected by the refractory coatings according to the invention which are just as advantageous at these lower temperatures.